Electrolytic capacitor

ABSTRACT

An electrolytic capacitor includes a capacitor element and electrolytic solution. The capacitor element includes an anode body with an oxide film, and a solid electrolyte contacting the oxide film. The electrolytic solution contains a solvent and a solute. The solvent contains at least one selected from the group consisting of a lactone compound, a glycol compound, and a sulfone compound. The solute includes a first acid component and a base component. The first acid component includes at least one of a benzenedicarboxylic acid and a derivative of the benzenedicarboxylic acid. The base component includes at least one of an amine and an amidine. A concentration of the solute in the electrolytic solution ranges from 15% by mass to 40% by mass, inclusive. A ratio (V/Vw) of a formation voltage V of the oxide film to a rated voltage Vw of the electrolytic capacitor is less than or equal to 1.7.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/810,105, filed on Mar. 5, 2020, which is a Continuation of the PCTInternational Application No. PCT/JP2018/036209 filed on Sep. 28, 2018,which claims the benefit of foreign priority of Japanese patentapplication No. 2017-191961 filed on Sep. 29, 2017, the contents all ofwhich are incorporated herein by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to an electrolytic capacitor including asolid electrolyte and an electrolytic solution.

2. Description of the Related Art

As a small-sized, high capacitance, and low equivalent series resistance(ESR) capacitor, a promising candidate is a so-called hybridelectrolytic capacitor, which is a capacitor including a solidelectrolyte and an electrolytic solution. For example, InternationalPublication No. 2011/099261 discloses a hybrid electrolytic capacitorincluding an anode body on which an oxide film (anodization film) isformed.

SUMMARY

An electrolytic capacitor according to a first aspect of the presentdisclosure includes a capacitor element and an electrolytic solution.The capacitor element includes an anode body with an oxide film, and asolid electrolyte in contact with the oxide film. The electrolyticsolution contains a solvent and a solute. The solvent contains at leastone selected from a group consisting of a lactone compound, a glycolcompound, and a sulfone compound. The solute includes a first acidcomponent and a base component. The first acid component includes atleast one of a benzenedicarboxylic acid and a derivative of thebenzenedicarboxylic acid. The base component includes at least one of anamine and an amidine. A concentration of the solute in the electrolyticsolution ranges from 15% by mass to 40% by mass, inclusive. A ratio(V/Vw) of a formation voltage V of the oxide film to a rated voltage Vwof the electrolytic capacitor is less than or equal to 1.7.

An electrolytic capacitor according to a second aspect of the presentdisclosure includes a capacitor element and an electrolytic solution.The capacitor element includes an anode body with an oxide film, and asolid electrolyte in contact with the oxide film. The electrolyticsolution contains a solvent and a solute. The solvent contains at leastone selected from a group consisting of a lactone compound, a glycolcompound, and a sulfone compound. The solute contains a first acidcomponent and a base component. The first acid component includes atleast one of a composite compound of an organic acid and an inorganicacid, and a derivative of the composite compound. The base componentincludes at least one of an amine and an amidine. A concentration of thesolute in the electrolytic solution ranges from 10% by mass to 40% bymass, inclusive. A ratio (V/Vw) of a formation voltage V of the oxidefilm to a rated voltage Vw of the electrolytic capacitor is less than orequal to 1.7.

According to the present disclosure, there can be provided a hybridelectrolytic capacitor capable of sufficiently retaining electrostaticcapacity and equivalent series resistance (ESR).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an electrolyticcapacitor according to one exemplary embodiment of the presentdisclosure; and

FIG. 2 is a schematic view for illustrating a configuration of acapacitor element according to the exemplary embodiment.

DETAILED DESCRIPTION OF EMBODIMENT

Conventional hybrid electrolytic capacitors have not been capable ofsufficiently retaining the electrostatic capacity and the equivalentseries resistance (ESR) when the ratio of the formation voltage to therated voltage is lowered.

First Exemplary Embodiment

An electrolytic capacitor according to the present exemplary embodimentincludes a capacitor element and an electrolytic solution. The capacitorelement includes an anode body including an oxide film, and a solidelectrolyte in contact with the oxide film.

In a hybrid electrolytic capacitor, a solid electrolyte is in directcontact with an oxide film. Accordingly, in order to reduce a leakcurrent, a formation voltage V of the oxide film is conventionally setat a value as high as about two times the rated voltage Vw of theelectrolytic capacitor to form a sufficiently thick oxide film. Thus, ithas been difficult, in the hybrid electrolytic capacitor, to reduce theratio (V/Vw) of the formation voltage V to the rated voltage Vw for thepurpose of an increase in the electrostatic capacity of the electrolyticcapacitor or a reduction of the size of the electrolytic capacitor.

As a result of earnest studies made by the inventors of the presentdisclosure, it has been found that the electrolytic solution having aspecific composition enables the hybrid electrolytic capacitor to retainthe electrostatic capacity and the ESR even when the ratio (V/Vw) of theformation voltage V to the rated voltage Vw is reduced.

First point is using a specific solute and setting a concentration ofthe solute in a specific range. Specifically, a solute that includes afirst acid component and a base component is used. As the first acidcomponent, at least one of a benzenedicarboxylic acid and a derivativeof the benzenedicarboxylic acid is used. As a base component, at leastone of an amine and an amidine is used. A concentration of the solutecontained in the electrolytic solution, i.e., a concentration of a totalof an acid component, which includes the first acid component, and thebase component is set at more than or equal to 15% by mass and less than40% by mass.

Second point is using a specific solvent. Specifically, as the solvent,at least one selected from the group consisting of γ-butyrolactone,ethylene glycol, and sulfolane is used.

In the electrolytic solution having the above composition, the firstacid component included in the solute is easily able to reach a vicinityof a defect of the anode body. Thus, self-repairing performance of theoxide film is improved, and the electrostatic capacity and the ESR ofthe electrolytic capacitor can be retained. Accordingly, the ratio(V/Vw) of the formation voltage V to the rated voltage Vw can be set atless than or equal to 1.7.

Meanwhile, when the electrolytic solution contains a polymer component,a concentration of the polymer component is set to range from 1% by massto 15% by mass, inclusive. In this range of the concentration of thepolymer component, even when the electrolytic solution contains thepolymer component, movement of the first acid component in theelectrolytic solution is less likely to be inhibited by the polymercomponent, so that the above effects can be obtained.

The oxide film is not limited to a film formed by a method (hereinafter,a first method) of applying a predetermined formation voltage to theanode body while immersing the anode body in an acidic aqueous solution(hereinafter, an anodizing solution). For example, the oxide film may beformed by heat-treating the anode body while immersing the anode body inan anodizing solution (hereinafter, a second method). When the oxidefilm is formed by the first method, the oxide film that has a thicknessT corresponding to the formation voltage is formed. That is, theformation voltage is specified by the thickness T of the oxide film.When the oxide film is formed by the second method, the formationvoltage necessary for forming the oxide film by the first method isspecified by the thickness T of the oxide film. That is, the formationvoltage V is defined as a voltage applied to the anode body to form theoxide film having the thickness T, as well as a voltage necessary forforming the oxide film having the thickness T.

The rated voltage Vw is an upper limit voltage set as rating, and amaximum value of voltage applied between electrodes of the electrolyticcapacitor.

<Electrolytic Solution>

The electrolytic solution contains a solvent and a solute.

The electrolytic solution preferably has a pH of less than or equal to4.5. The electrolytic solution having a pH of less than or equal to 4.5easily suppresses a dedoping phenomenon of the solid electrolyte. Thesuppression of the dedoping phenomenon enables the electrolyticcapacitor to retain the ESR. The electrolytic solution has a pH offurther preferably less than or equal to 4, particularly preferably lessthan or equal to 3.8. The electrolytic solution preferably has a pH ofmore than or equal to 2.

The electrolytic solution preferably has a conductivity ranging from0.01 mS/cm to 3 mS/cm, inclusive. The electrolytic solution having aconductivity in the above range further easily improves theself-repairing performance of the oxide film when the ratio (V/Vw) ofthe formation voltage V to the rated voltage Vw is set at less than orequal to 1.7.

(Solvent)

The solvent preferably contains at least one (hereinafter, a mainsolvent) selected from the group consisting of γ-butyrolactone (γBL),ethylene glycol (EG), and sulfolane (SL). As the main solvent, there maybe used a glycol compound other than EG, a sulfone compound other thanSL, and a lactone compound other than γBL. As the glycol compound otherthan EG, there can be used, for example, diethylene glycol, triethyleneglycol, and propylene glycol. As the sulfone compound other than SL,there can be used, for example, dimethyl sulfoxide and diethylsulfoxide. As the lactone compound other than γBL, there can be used,for example, γ-valerolactone. A proportion of the main solvent (forexample, a total of γBL, EG, and SL) in the solvent is preferably morethan or equal to 50% by mass, further preferably more than or equal to60% by mass, further preferably more than or equal to 70% by mass.

The solvent can contain, as a solvent (hereinafter, an auxiliarysolvent) other than the main solvent, a carbonate compound, a monohydricor trihydric or higher alcohol, or the like. As the carbonate compound,there can be used, for example, dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC),propylene carbonate (PC), and fluoroethylene carbonate (FEC). As thealcohol, there can be used, for example, glycerin and polyglycerin.These compounds may be used alone or in combination of a plurality ofcompounds.

(Solute)

The concentration of the solute ranges from 15% by mass to 40% by mass,inclusive. The concentration of the solute ranges further preferablyfrom 20% by mass to 40% by mass, inclusive, particularly preferably from20% by mass to 35% by mass, inclusive.

A concentration of the solute is a total of a concentration of an acidcomponent and a concentration of a base component. The acid componentincludes a first acid component and a second acid component other thanthe first acid component. The base component includes an amine and/or anamidine (hereinafter, a first base component) and a second basecomponent other than the first base component.

The solute includes, as the first acid component, at least one of abenzenedicarboxylic acid and a derivative of the benzenedicarboxylicacid. The benzenedicarboxylic acid may be o-phthalic acid, m-phthalicacid, or p-phthalic acid. Examples of the derivative of thebenzenedicarboxylic acid include 3-sulfophthalic acid,3,5-disulfophthalic acid, 4-sulfoisophthalic acid, 2-sulfoterephthalicacid, and 2-methyl-5-sulfoterephthalic acid each having a sulfo group.Especially, o-phthalic acid is preferable.

A concentration of the first acid component in the electrolytic solutionis preferably more than or equal to 5% by mass, further preferably morethan or equal to 15% by mass, in consideration of easy disassociation ofthe first acid component. The concentration of the first acid componentis preferably less than or equal to 35% by mass, further preferably lessthan or equal to 30% by mass.

The acid component may include a second acid component other than thefirst acid component.

Examples of an organic acid used as the second acid component include apolycarboxylic acid, a monocarboxylic acid, and a polyphenol.

Examples of the polycarboxylic acid include aliphatic polycarboxylicacids: ([saturated polycarboxylic acids such as oxalic acid, malonicacid, succinic acid, glutaric acid, adipic acid, pimelic acid, subericacid, azelaic acid, sebacic acid, 1,6-decanedicarboxylic acid, and5,6-decanedicarboxylic acid] and [unsaturated polycarboxylic acids suchas maleic acid, fumaric acid, and itaconic acid]), aromaticpolycarboxylic acids: (such as trimellitic acid and pyromellitic acid),and alicyclic polycarboxylic acids: (such ascyclohexane-1,2-dicarboxylic acid and cyclohexene-1,2-dicarboxylicacid).

Examples of the monocarboxylic acid include aliphatic monocarboxylicacids (1 to 30 carbon atoms): ([saturated monocarboxylic acids such asformic acid, acetic acid, propionic acid, butyric acid, isobutyric acid,valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonicacid, lauric acid, myristic acid, stearic acid, and behenic acid] and[unsaturated monocarboxylic acids such as acrylic acid, methacrylicacid, and oleic acid]), aromatic monocarboxylic acids: (such as benzoicacid, cinnamic acid, and naphthoic acid), and oxycarboxylic acids: (suchas salicylic acid, mandelic acid, and resorcinol acid). Among thesepolycarboxylic acids and monocarboxylic acids, maleic acid, benzoicacid, pyromellitic acid, and resorcinol acid are preferable that havehigh conductivity and are thermally stable.

Examples of the polyphenol include catechol, resorcinol, hydroquinone,pyrogallol, and phloroglucin.

Examples of an inorganic acid used as the second acid component includea carbon compound, a hydrogen compound, a boron compound, a sulfurcompound, a nitrogen compound, and a phosphorus compound. Typicalexamples of the inorganic acid include phosphoric acid, phosphorousacid, hypophosphorous acid, alkyl phosphoric acid ester, boric acid,fluoroboric acid, tetrafluoroboric acid, hexafluorophosphoric acid,benzenesulfonic acid, and naphthalenesulfonic acid.

As the second acid component, there may be used a composite compound ofan organic acid and an inorganic acid. Examples of the compositecompound include borodiglycolic acid, borodioxalic acid, andborodisalicylic acid.

Especially, the second acid component is preferably at least oneselected from the group consisting of an aromatic polycarboxylic acid, apolyphenol, and an oxycarboxylic acid, in consideration of furtherimproving the self-repairing performance of the oxide film.

A concentration of the second acid component is preferably more than orequal to 3% by mass, further preferably more than or equal to 5% bymass. The concentration of the second acid component is preferably lessthan or equal to 25% by mass, further preferably less than or equal to15% by mass.

The solute contains, as the first base component, at least one of anamine or an amidine.

The amine may be a primary amine, a secondary amine, or a tertiaryamine. Each of the amines may be an aliphatic amine, an aromatic amine,or a heterocyclic amine. Especially, a tertiary amine is preferable inconsideration of enhancing an effect of stabilizing the ESR for a longperiod.

Examples of the tertiary amine include trialkylamines (such astrimethylamine, dimethylethylamine, methyldiethylamine, triethylamine,dimethyl-n-propylamine, dimethylisopropylamine,methylethyl-n-propylamine, methylethylisopropylamine,diethyl-n-propylamine, diethylisopropylamine, tri-n-propylamine,triisopropylamine, tri-n-butylamine, and tri-t-butylamine) and phenylgroup-containing amines (such as dimethylphenylamine,methylethylphenylamine, and diethylphenylamine). Especially,trialkylamines such as trimethylamine, dimethylethylamine,methyldiethylamine, and triethylamine are preferable in consideration ofhigh conductivity.

The amidine is preferably a compound having an alkyl-substituted amidinegroup, in consideration of high conductivity. Examples of the compoundhaving an alkyl-substituted amidine group include an imidazole compound,a benzimidazole compound, and an alicyclic amidine compound (apyrimidine compound and an imidazoline compound). Specific examples ofthe compound having an alkyl-substituted amidine group include1,8-diazabicyclo[5,4,0]undecene-7,1,5-diazabicyclo[4,3,0]nonene-5,1,2-dimethylimidazolinium,1,2,4-trimethylimidazoline, 1-methyl-2-ethylimidazoline,1,4-dimethyl-2-ethylimidazoline, 1-methyl-2-heptylimidazoline,1-methyl-2-(3′heptyl)imidazoline, 1-methyl-2-dodecylimidazoline,1,2-dimethyl-1,4,5,6-tetrahydropyrimidine, 1-methylimidazole,1-methylbenzimidazole,1-methyl-1,8-diazabicyclo[5,4,0]undecene-7,1-methyl-1,5-diazabicyclo[4,3,0]nonene-5,1,2,3-trimethylimidazolinium,1,2,3,4-tetramethylimidazolinium, 1,2-dimethyl-3-ethyl-imidazolinium,1,3,4-trimethyl-2-ethylimidazolinium,1,3-dimethyl-2-heptylimidazolinium,1,3-dimethyl-2-(3′heptyl)imidazolinium,1,3-dimethyl-2-dodecylimidazolinium,1,2,3-trimethyl-1,4,5,6-tetrahydropyrimidium, 1,3-dimethylimidazolium,1-methyl-3-ethylimidazolium, and 1,3-dimethylbenzimidazolium.

A concentration of the first base component in the electrolytic solutionis preferably more than or equal to 3.5% by mass, further preferablymore than or equal to 5% by mass. The concentration of the first basecomponent is preferably less than or equal to 20% by mass, furtherpreferably less than or equal to 10% by mass.

When the electrolytic solution includes an amine, a concentration of theamine in the electrolytic solution is preferably more than or equal to3.5% by mass, further preferably more than or equal to 5% by mass. Theconcentration of the amine is preferably less than or equal to 20% bymass, further preferably less than or equal to 10% by mass.

When the electrolytic solution includes an amidine, a concentration ofthe amidine in the electrolytic solution is preferably more than orequal to 3.5% by mass, further preferably more than or equal to 5% bymass. The concentration of the amidine is preferably less than or equalto 20% by mass, further preferably less than or equal to 10% by mass.

The base component may include a second base component other than thefirst base component.

Examples of the second base component include ammonia and a quaternaryammonium compound. A concentration of the second base component in theelectrolytic solution is preferably more than or equal to 0.1% by mass,further preferably more than or equal to 3% by mass. The concentrationof the second base component is preferably less than or equal to 20% bymass, further preferably less than or equal to 10% by mass.

From a viewpoint of effectively suppressing dedoping of a dopantcontained in a conductive polymer, the acid component is preferably moreexcessive than the base component in terms of an equivalence ratio. Forexample, the equivalence ratio of the acid component to the basecomponent preferably ranges from 1 to 30, inclusive.

(Polymer Component)

The electrolytic solution may contain a polymer component. The polymercomponent is contained to suppress evaporation of the electrolyticsolution and to improve withstand voltage of the electrolytic capacitor.

The polymer component is not particularly limited. Examples of thepolymer component include a polyalkylene glycol, a derivative of thepolyalkylene glycol, and a compound obtained by substituting at leastone hydroxy group of a polyol with a polyalkylene glycol (including aderivative). Specific examples of the polymer component includepolyethylene glycol, polyethylene glycol glyceryl ether, polyethyleneglycol diglyceryl ether, polyethylene glycol sorbitol ether,polypropylene glycol, polypropylene glycol glyceryl ether, polypropyleneglycol diglyceryl ether, polypropylene glycol sorbitol ether, andpolybutylene glycol. These polymer components may be used alone or inmixture of two or more polymer components.

The polyalkylene glycol may be a copolymer (a random copolymer, a blockcopolymer, a random block copolymer, or the like). Examples of thecopolymer include a copolymer of ethylene glycol and propylene glycol, acopolymer of an ethylene glycol and butylene glycol, and a copolymer ofpropylene glycol and butylene glycol.

The polymer component preferably has a weight-average molecular weightof more than or equal to 200. From a viewpoint of solubility in thesolvent, the polymer component has a weight-average molecular weight ofpreferably less than or equal to 20,000, further preferably less than orequal to 5000.

A concentration of the polymer component in the electrolytic solutionpreferably ranges from 1% by mass to 15% by mass, inclusive. When theelectrolytic solution has a concentration of the polymer component inthe above range, evaporation of the electrolytic solution is suppressed,as well as movement of the first acid component in the electrolyticsolution is not inhibited. Accordingly, the self-repairing performanceof the oxide film is improved. The concentration of the polymercomponent in the electrolytic solution further preferably ranges from 1%by mass to 10% by mass, inclusive.

(Solid Electrolyte)

The solid electrolyte contains, for example, a manganese compound and aconductive polymer. As the conductive polymer, there can be used, forexample, polypyrrole, polythiophene, polyaniline, and derivatives ofthese polymers. The solid electrolyte contains a dopant. Morespecifically, the solid electrolyte includes poly(3,4-ethylenedioxythiophene) (PEDOT) as the conductive polymer andpolystyrenesulfonic acid (PSS) as the dopant.

The solid electrolyte may be formed by a method for applying, to theoxide film, a solution containing a monomer and the dopant, to causechemical polymerization or electrolytic polymerization on the oxidefilm. The solid electrolyte, however, is preferably formed by a methodfor applying the conductive polymer to the oxide film, in considerationof excellent withstand voltage characteristics of the electrolyticcapacitor expected by this method. That is, the solid electrolyte ispreferably one formed by impregnating the oxide film with a polymerdispersion including a liquid component, the conductive polymer, and thedopant, and then volatilizing the liquid component. Here, the conductivepolymer and the dopant are dispersed in the liquid component.

A concentration of the conductive polymer in the polymer dispersionpreferably ranges from 0.5% by mass to 10% by mass, inclusive. Theconductive polymer preferably has an average particle diameter D50ranging, for example, from 0.01 μm to 0.5 μm inclusive. Here, theaverage particle diameter D50 is a median diameter in a volume particlesize distribution obtained by a particle size distribution measuringapparatus according to dynamic light scattering.

(Formation Voltage V and Rated Voltage Vw)

The ratio (V/Vw) of the formation voltage V to the rated voltage Vw ofthe electrolytic capacitor is less than or equal to 1.7. Here, theformation voltage V is a voltage applied to the anode body for formingthe oxide film having the thickness T. The ratio V/Vw may be less thanor equal to 1.6. From a viewpoint of suppressing an increase of the leakcurrent, the ratio V/Vw is preferably more than or equal to 1.4, furtherpreferably more than or equal to 1.5.

The formation voltage V is not particularly limited, and may beappropriately set to give a ratio V/Vw of less than or equal to 1.7according to the rated voltage Vw. The thickness T of the oxide filmincreases in proportion to the formation voltage V. For example, aformation voltage V of 17 volts forms the oxide film having a thicknessT of 24 nm. A formation voltage V of 170 volts forms the oxide filmhaving a thickness T of 238 nm. In other words, the formation voltage Vapplied or necessary to be applied to the anode body is specified to be170 volts by the oxide film having a thickness T of 238 nm.

The rated voltage Vw is not also particularly limited, but the effectsof the present disclosure are particularly exhibited when the ratedvoltage Vw is less than or equal to 100 V (that is, when the oxide filmhas a thickness T of less than or equal to 238 nm). Particularly, whenthe rated voltage Vw is less than or equal to 70 volts that forms athinner oxide film, the effects of the present disclosure are furtherexhibited.

Hereinafter, the present disclosure is more specifically described withreference to the exemplary embodiment. The exemplary embodimentdescribed below, however, is not to limit the present disclosure.

FIG. 1 is a schematic cross-sectional view illustrating an electrolyticcapacitor according to the present exemplary embodiment, and FIG. 2 is aschematic view in which a capacitor element of the electrolyticcapacitor is partially developed.

The electrolytic capacitor includes, for example, capacitor element 10,bottomed case 11 housing capacitor element 10, sealing member 12 sealingan opening of bottomed case 11, seat plate 13 that covers sealing member12. The electrolytic capacitor further includes lead wires 14A, 14B thatare drawn out from sealing member 12 and penetrate through seat plate13, lead tabs 15A, 15B connecting the lead wires to electrodes ofcapacitor element 10, and an electrolytic solution (not shown). Bottomedcase 11 is, at a part near an opening end, drawn inward, and is, at theopening end, curled to swage sealing member 12.

Capacitor element 10 is produced from a wound body illustrated in FIG. 2. The wound body includes anode body 21 connected to lead tab 15A,cathode body 22 connected to lead tab 15B, and separator 23. The woundbody is a half-finished product where no solid electrolyte is formedbetween anode body 21 and cathode body 22.

Anode body 21 and cathode body 22 are wound with separator 23 interposedbetween the anode body and the cathode body. An outermost periphery ofthe wound body is fixed with fastening tape 24. FIG. 2 illustrates thewound body that is in a state before the outermost periphery of thewound body is fixed and that is partially developed.

Anode body 21 includes a metal foil whose surface is roughened to haveprojections and recesses, and an oxide film is formed on the metal foilhaving the projections and recesses. A solid electrolyte is attached toat least a part of a surface of the oxide film. The solid electrolytemay also cover at least a part of a surface of cathode body 22 and/or apart of a surface of separator 23. Capacitor element 10 in which thesolid electrolyte has been formed is housed in bottomed case 11 togetherwith the electrolytic solution.

<<Method for Manufacturing Electrolytic Capacitor>>

Hereinafter, described are steps of one exemplary method formanufacturing the electrolytic capacitor according to the presentexemplary embodiment.

(i) Step of Preparing Anode Body 21 Including Oxide Film

First, a metal foil as a raw material for anode body 21 is prepared. Atype of the metal is not particularly limited, but it is preferred touse a valve metal such as aluminum, tantalum, or niobium, or an alloyincluding a valve metal, in consideration of facilitating formation ofthe oxide film.

Next, a surface of the metal foil is roughened. By the roughening, aplurality of projections and recesses are formed on the surface of themetal foil. The roughening is preferably performed by subjecting themetal foil to an etching treatment. The etching treatment may beperformed by, for example, a direct current (DC) electrolytic method oran alternating current (AC) electrolytic method.

Next, an oxide film having a thickness T is formed on the roughenedsurface of the metal foil. A method for forming the oxide film is notparticularly limited, and the oxide film can be formed by subjecting themetal foil to an anodizing treatment. The anodizing treatment isperformed by, for example, immersing the metal foil in an anodizingsolution such as an ammonium adipate solution, followed by a heattreatment. The anodizing treatment may also be performed by applying avoltage to the metal foil that has been immersed in the anodizingsolution.

Normally, a large foil of, for example, a valve metal (metal foil) issubjected to the roughening treatment and the anodizing treatment from aviewpoint of mass productivity. In this case, the treated foil is cut ina desired size to prepare anode body 21.

(ii) Step of Preparing Cathode Body 22

A metal foil can be used for cathode body 22 as with anode body 21. Atype of the metal is not particularly limited, but it is preferred touse a valve metal such as aluminum, tantalum, or niobium, or an alloyincluding a valve metal. A surface of cathode body 22 may be roughenedas necessary.

In addition, a layer containing titanium or carbon may be formed on thesurface of cathode body 22.

(iii) Production of Wound Body

Next, anode body 21, cathode body 22, and separator 23 are used toproduce a wound body illustrated in FIG. 2 . An end of cathode body 22positioned at an outermost layer is fixed with fastening tape 24. Whenanode body 21 is prepared by cutting a large metal foil, the wound bodymay further be subjected to an anodizing treatment in order to providethe oxide film on a cutting surface of anode body 21. As separator 23, anonwoven fabric can be used that contains, as a main component, forexample, a cellulose, polyethylene terephthalate, vinylon, or aramidfiber.

(iv) Step of Forming Capacitor Element 10

Next, a solid electrolyte is attached to a surface of the oxide filmincluded in the wound body, to produce capacitor element 10. When thesolid electrolyte includes a conductive polymer, a polymerization liquidmay be used to cause chemical polymerization or electrolyticpolymerization on the oxide film and thus to attach a synthesizedconductive polymer to the oxide film. The polymerization liquid is asolution containing a monomer or an oligomer, a dopant, and the like. Inthe case of chemical polymerization, an oxidant is added to thepolymerization liquid. Alternatively, a conductive polymer synthesizedin advance may be attached to the oxide film. Pyrrole, aniline,thiophene, or a derivative of pyrrole, aniline, or thiophene is used forthe monomer or the oligomer.

It is preferred to use a polymer dispersion as the conductive polymersynthesized in advance. The polymer dispersion contains a liquidcomponent, a conductive polymer, and a dopant. The conductive polymerand the dopant are dispersed in the liquid component. A method forapplying the polymer dispersion to the surface of the oxide film ispreferably, for example, a method for impregnating the wound body withthe polymer dispersion and drying the wound body, because the method issimple. The polymer dispersion preferably contains PEDOT as theconductive polymer and PSS as the dopant.

The step of applying the polymer dispersion to the surface of the oxidefilm and the step of drying the wound body may be repeated two or moretimes. These steps can be performed a plurality of times to increasecoverage of the solid electrolyte on the oxide film.

(v) Step of Impregnating Capacitor Element 10 with Electrolytic Solution

Next, capacitor element 10 is impregnated with an electrolytic solution.A method for impregnating capacitor element 10 with the electrolyticsolution is not particularly limited.

(vi) Step of Sealing Capacitor Element in Bottomed Case

Next, capacitor element 10 is housed in bottomed case 11. As a materialfor bottomed case 11, there can be used metals such as aluminum,stainless steel, copper, iron and brass, and alloys of these metals.Thereafter, bottomed case 11 is, at a part near an opening end,laterally drawn, and is, at the opening end, curled to swage sealingmember 12. Then, seat plate 13 is disposed on a curled part of thebottomed case to complete an electrolytic capacitor illustrated in FIG.1 . Then, an aging treatment may be performed while a rated voltage isapplied.

In the exemplary embodiment described above, a wound electrolyticcapacitor has been described. An application range of the presentdisclosure, however, is not limited to the wound electrolytic capacitor,and the present disclosure is also applicable to other electrolyticcapacitors such as a chip electrolytic capacitor including a metalsintered body as an anode body, and a laminated electrolytic capacitorincluding a metal plate as an anode body.

Second Exemplary Embodiment

An electrolytic capacitor according to a second exemplary embodiment ofthe present disclosure is described. The electrolytic capacitoraccording to the second exemplary embodiment has a same configuration asthe electrolytic capacitor according to the first exemplary embodimentexcept that the solute contains, as the first acid component, at leastone of a composite compound of an organic acid and an inorganic acid,and a derivative of the composite compound. It is noted that the samecontents as the first exemplary embodiment are not described.

In this present exemplary embodiment, the composite compound serving asthe first acid component preferably includes at least one selected fromthe group consisting of borodisalicylic acid, borodiglycolic acid, andborodioxalic acid.

The concentration of the solute included in the electrolytic solution,i.e., the concentration of the total of the acid component including thefirst acid component, and the base component is set at more than orequal to 10% by mass and less than 40% by mass. The concentration of thesolute ranges further preferably from 15% by mass to 35% by mass,inclusive, particularly preferably from 20% by mass to 35% by mass,inclusive. Since the composite compound used as the first acid componenthas a high degree of disassociation in the electrolytic solution, thefirst acid component easily reaches a vicinity of a defect of the anodebody even when the concentration of the solute is more than or equal to10% by mass. Thus, similarly to the first exemplary embodiment, theself-repairing performance of the oxide film can be improved. Further,since the composite compound has excellent heat resistance, pH of theelectrolytic solution containing the composite compound can be easilyretained. Hence, the dedoping of the dopant from the conductive polymercan be suppressed, and thus ESR of the electrolytic capacitor can beretained.

EXAMPLES

Hereinafter, the present disclosure is described in more detail withreference to examples. The present disclosure, however, is not to belimited to the examples.

Example 1

A wound electrolytic capacitor ((diameter) Φ 10 mm×(length) L 10 mm)with a rated voltage Vw of 25 volts and a rated electrostatic capacityof 33 μF was produced in the following manner.

(Preparation of Anode Body)

A 100-μm-thick aluminum foil was subjected to an etching treatment toroughen a surface of the aluminum foil. The roughened surface of thealuminum foil was subjected to an anodizing treatment to form an oxidefilm. The anodizing treatment was performed by immersing the aluminumfoil in an ammonium adipate solution and applying a voltage of 40 voltsto the aluminum foil. Thereafter, the aluminum foil was cut in a size of6 mm×120 mm to prepare an anode body. The ratio V/Vw was set at 1.6. Theoxide film had a thickness T of 55 nm.

(Preparation of Cathode Body)

A 50-μm-thick aluminum foil was subjected to an etching treatment toroughen a surface of the aluminum foil. Thereafter, the aluminum foilwas cut in a size of 6 mm×120 mm to prepare a cathode body.

(Production of Wound Body)

An anode lead tab and a cathode lead tab were connected to the anodebody and the cathode body, respectively, and the anode body and thecathode body were wound with a separator interposed between the anodebody and the cathode body while the lead tabs were rolled in the anodebody, the cathode body, and the separator. Ends of the lead tabsprotruding from the wound body were connected to an anode lead wire anda cathode lead wire, respectively. The wound body obtained was anodizedagain to form an oxide film at a cutting end of the anode body. An endof an outer surface of the wound body was fixed with a fastening tape.

(Preparation of Polymer Dispersion)

A mixed solution was prepared by dissolving 3,4-ethylenedioxythiopheneand polystyrenesulfonic acid (PSS, weight-average molecular weight of100000) in ion-exchanged water. While the mixed solution was stirred,iron(III) sulfate (oxidant) was added to the mixed solution to cause apolymerization reaction. Thereafter, the reaction solution was dialyzedto remove unreacted monomers and the oxidant, so that a polymerdispersion was obtained that contained about 5% by mass ofpolyethylenedioxythiophene doped with PSS (PEDOT/PSS).

(Formation of Solid Electrolyte Layer)

The wound body was immersed in the polymer dispersion in areduced-pressure atmosphere (40 kPa) for 5 minutes, and then the woundbody was picked up from the polymer dispersion. Next, the wound bodythat had been impregnated with the polymer dispersion was dried in adrying furnace at 150° C. for 20 minutes to form a solid electrolytecovering at least a part of the oxide film.

(Impregnation with Electrolytic Solution)

As a solvent, γ-butyrolactone (γBL) and sulfolane were prepared. In thissolvent, o-phthalic acid as the first acid component and triethylamineas the first base component were dissolved at a concentration of 19% bymass in total of the first acid component and the first base component.An equivalence ratio (initial equivalence ratio) of the first acidcomponent to the first base component is was set to be 1. Polyethyleneglycol (PEG) (weight-average molecular weight of 300) at a concentrationof 10% by mass was dissolved in the solution obtained above. Lastly, 12%by mass of o-phthalic acid was added, and 3% by mass of pyrogallol wasalso added to prepare an electrolytic solution having a pH of 3.5. Thecapacitor element was immersed in the electrolytic solution in areduced-pressure atmosphere (40 kPa) for 5 minutes. The concentration ofeach component is a concentration when a mass of the electrolyticsolution obtained was defined as 100%. The concentration of the acidcomponent was 28.2% by mass, and the concentration of the base componentwas 5.8% by mass.

(Sealing of Capacitor Element in Bottomed Case)

The capacitor element that had been impregnated with the electrolyticsolution was sealed in a bottomed case to complete an electrolyticcapacitor (A1) illustrated in FIG. 1 . Thereafter, aging was performedat 95° C. for 90 minutes while applying the rated voltage Vw.

<Evaluation>

An electrostatic capacity and ESR of the electrolytic capacitor A1 weremeasured after the aging and after 2500 hours. Values of theelectrostatic capacity and the ESR after 2500 hours were respectivelydivided by values of the electrostatic capacity and the ESR after theaging in order to calculate change rates. Table 1 shows results.

Examples 2 to 5

Electrolytic capacitors A2 to A5 were produced in the same manner as inExample 1 except that the concentration of PEG was changed as shown inTable 1. The electrolytic capacitors A2 to A5 were evaluated in the samemanner. Table 1 shows results.

Comparative Example 1

An electrolytic capacitor B1 was produced in the same manner as inExample 1 except that the initial concentration of the solute was set at10% by mass without changing the initial equivalence ratio of the firstacid component to the first base component, the concentration of theadditional o-phthalic acid was set at 4% by mass, and no pyrogallol wasadded (the concentration of the total solute was set at 14% by mass).The electrolytic capacitor B1 was evaluated in the same manner. Table 1shows results. The concentration of the acid component was 10.9% bymass, and the concentration of the base component was 3.1% by mass.

Comparative Example 2

An electrolytic capacitor B2 was produced in the same manner as inComparative Example 1 except that the formation voltage V was set at 45volts to give a ratio V/Vw of 1.8. The electrolytic capacitor B2 wasevaluated in the same manner. Table 1 shows results.

TABLE 1 Concentration of solute (% by mass) Concentration of Total ofAdditional second acid component Concentration Change rate (%)Electrolytic initially added o-phthalic Pyromellitic of PEGElectrostatic capacitor V/Vw components acid acid Pyrogallol (% by mass)pH capacity ESR A1 1.6 19 12 — 3 10 3.5 6.0 1.4 A2 1.6 19 12 — 3 — 3.54.9 1.3 A3 1.6 19 12 — 3 1 3.5 5.0 1.4 A4 1.6 19 12 — 3 5 3.5 5.3 1.4 A51.6 19 12 — 3 15 3.5 6.2 1.4 B1 1.6 10 4 — — 10 4.3 37.0 18 B2 1.8 10 4— — 10 4.3 5.2 1.4

Example 6

An electrolytic capacitor A6 was produced in the same manner as inExample 1 except that 4% by mass of pyromellitic acid and 5% by mass ofpyrogallol were added in place of the additional o-phthalic acid (12% bymass) and the concentration of PEG was set at 15% by mass. Theelectrolytic capacitor A6 was evaluated in the same manner. Table 2shows results.

Example 7

An electrolytic capacitor A7 was produced in the same manner as inExample 1 except that the initial concentration of the solute was set at12% by mass without changing the initial equivalence ratio of the firstacid component to the first base component, and 3% by mass ofpyromellitic acid was added in place of pyrogallol. The electrolyticcapacitor A7 was evaluated in the same manner. Table 2 shows results.

Example 8

An electrolytic capacitor A8 was produced in the same manner as inExample 1 except that the initial concentration of the solute was set at10% by mass without changing the initial equivalence ratio of the firstacid component to the first base component, and no pyrogallol was added.The electrolytic capacitor A8 was evaluated in the same manner. Table 2shows results.

Example 9

An electrolytic capacitor A9 was produced in the same manner as inExample 1 except that the concentration of the additional o-phthalicacid was set at 6% by mass. The electrolytic capacitor A9 was evaluatedin the same manner. Table 2 shows results.

Example 10

An electrolytic capacitor A10 was produced in the same manner as inExample 1 except that the initial concentration of the solute was set at25% by mass without changing the initial equivalence ratio of the firstacid component to the first base component, the concentration of theadditional o-phthalic acid was set at 10% by mass, and the concentrationof pyrogallol was set at 5% by mass. The electrolytic capacitor A10 wasevaluated in the same manner. Table 2 shows results.

TABLE 2 Concentration of solute (% by mass) Concentration of Total ofAdditional second acid component Concentration Change rate (%)Electrolytic initially added o-phthalic Pyromellitic of PEGElectrostatic capacitor V/Vw components acid acid Pyrogallol (% by mass)pH capacity ESR A6 1.6 19 — 4 5 15 3.7 6.3 1.4 A7 1.6 12 — 3 — 10 4.58.0 1.6 A8 1.6 10 12 — — 10 3.7 6.5 1.5 A9 1.6 19  6 — 3 10 4.0 6.0 1.5A10 1.6 25 10 — 5 10 3.0 5.0 1.4

Example 11

An electrolytic capacitor A11 was produced in the same manner as inExample 1 except that the formation voltage V was set at 35 volts togive a ratio V/Vw of 1.4 and no pyrogallol was added. The electrolyticcapacitor A11 was evaluated in the same manner. Table 3 shows results.

Comparative Example 3

An electrolytic capacitor B3 was produced in the same manner as inComparative Example 1 except that the formation voltage V was set at 35volts to give a ratio V/Vw of 1.4. The electrolytic capacitor B3 wasevaluated in the same manner. Table 3 shows results.

TABLE 3 Concentration of solute (% by mass) Concentration of Total ofAdditional second acid component Concentration Change rate (%)Electrolytic initially added o-phthalic Pyromellitic of PEGElectrostatic capacitor V/Vw components acid acid Pyrogallol (% by mass)pH capacity ESR A11 1.4 19 12 — — 10 3.7  50  33 B3 1.4 10  4 — — 10 4.3200 300

Example 12

An electrolytic capacitor A12 was produced in the same manner as inExample 1 except that in place of triethylamine, an amidine, which is1,2,3,4-tetramethylimidazolinium, was used as the first base componentand the initial concentration of the solute was set at 14% by masswithout changing the initial equivalence ratio of the first acidcomponent to the first base component. The electrolytic capacitor A12was evaluated in the same manner. The concentration of the acidcomponent was 22.2% by mass, and the concentration of the base componentwas 6.8% by mass. Table 4 shows results.

Example 13

An electrolytic capacitor A13 was produced in the same manner as inExample 12 except that no PEG was added. The electrolytic capacitor A13was evaluated in the same manner. Table 4 shows results.

TABLE 4 Concentration of solute (% by mass) Concentration of Total ofAdditional second acid component Concentration Change rate (%)Electrolytic initially added o-phthalic Pyromellitic of PEGElectrostatic capacitor V/Vw components acid acid Pyrogallol (% by mass)pH capacity ESR A12 1.6 14 12 — 3 10 3.5 5.5 1.4 A13 1.6 14 12 — 3 — 3.54.8 1.3

The present disclosure is applicable to a hybrid electrolytic capacitorincluding a solid electrolyte and an electrolytic solution.

What is claimed is:
 1. An electrolytic capacitor comprising: a capacitorelement; and an electrolytic solution, wherein: the capacitor elementincludes an anode body with an oxide film and a solid electrolyte incontact with the oxide film, the electrolytic solution contains asolvent and a solute, the solvent contains at least one selected from agroup consisting of a lactone compound, a glycol compound, and a sulfonecompound, the solute includes a first acid component and a basecomponent, the first acid component including at least one of abenzenedicarboxylic acid and a derivative of the benzenedicarboxylicacid, the base component including at least one of an amine and anamidine, a concentration of the solute in the electrolytic solutionranges from 15% by mass to 40% by mass, inclusive, and a ratio V/Vw of aformation voltage V of the oxide film to a rated voltage Vw of theelectrolytic capacitor is more than or equal to 1.5 and less than orequal to 1.7.
 2. The electrolytic capacitor according to claim 1,wherein the benzenedicarboxylic acid is o-phthalic acid.
 3. Theelectrolytic capacitor according to claim 1, wherein a pH of theelectrolytic solution is less than or equal to 4.5.
 4. The electrolyticcapacitor according to claim 1, wherein: a conductivity of theelectrolytic solution ranges from 0.01 mS/cm to 3 mS/cm, inclusive. 5.The electrolytic capacitor according to claim 1, wherein the ratedvoltage Vw is less than or equal to 100 volts.
 6. The electrolyticcapacitor according to claim 1, wherein a concentration of the basecomponent in the electrolytic solution is more than or equal to 3.5% bymass.
 7. The electrolytic capacitor according to claim 1, wherein: thesolute further includes a second acid component other than the firstacid component, and a concentration of the second acid component in theelectrolytic solution is more than or equal to 3% by mass.
 8. Theelectrolytic capacitor according to claim 1, wherein the lactonecompound is γ-butyrolactone.
 9. The electrolytic capacitor according toclaim 1, wherein the glycol compound is ethylene glycol.
 10. Theelectrolytic capacitor according to claim 1, wherein the sulfonecompound is sulfolane.
 11. An electrolytic capacitor comprising: acapacitor element; and an electrolytic solution, wherein: the capacitorelement includes an anode body with an oxide film and a solidelectrolyte in contact with the oxide film, the electrolytic solutioncontains a solvent and a solute, the solvent contains at least oneselected from a group consisting of a lactone compound, a glycolcompound, and a sulfone compound, the solute includes a first acidcomponent and a base component, the first acid component including atleast one of a composite compound of an organic acid and an inorganicacid and a derivative of the composite compound, the base componentincluding at least one of an amine and an amidine, a concentration ofthe solute in the electrolytic solution ranges from 10% by mass to 40%by mass, inclusive, and a ratio V/Vw of a formation voltage V of theoxide film to a rated voltage Vw of the electrolytic capacitor is morethan or equal to 1.5 and less than or equal to 1.7.
 12. The electrolyticcapacitor according to claim 11, wherein a pH of the electrolyticsolution is less than or equal to 4.5.
 13. The electrolytic capacitoraccording to claim 11, wherein: a conductivity of the electrolyticsolution ranges from 0.01 mS/cm to 3 mS/cm, inclusive.
 14. Theelectrolytic capacitor according to claim 11, wherein the rated voltageVw is less than or equal to 100 volts.
 15. The electrolytic capacitoraccording to claim 11, wherein a concentration of the base component inthe electrolytic solution is more than or equal to 3.5% by mass.
 16. Theelectrolytic capacitor according to claim 11, wherein: the soluteincludes a second acid component other than the first acid component,and a concentration of the second acid component in the electrolyticsolution is more than or equal to 3% by mass.
 17. The electrolyticcapacitor according to claim 11, wherein the lactone compound isγ-butyrolactone.
 18. The electrolytic capacitor according to claim 11,wherein the glycol compound is ethylene glycol.
 19. The electrolyticcapacitor according to claim 11, wherein the sulfone compound issulfolane.